Discovering the Local Location Information Server (LIS)
draft-ietf-geopriv-lis-discovery-13

Abstract

Discovery of the correct Location Information Server (LIS) in the local access network is necessary for devices that wish to acquire location information from the network. A method is described for the discovery of a LIS in the access network serving a device. Dynamic Host Configuration Protocol (DHCP) options for IP versions 4 and 6 are defined that specify a domain name. This domain name is then used as input to a URI-enabled NAPTR (U-NAPTR) resolution process.

Status of This Memo

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Copyright Notice

Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1.  Introduction and Overview
    1.1.  Discovery Procedure Overview
    1.2.  Terminology
2.  LIS Discovery Procedure
    2.1.  Residential Gateways
    2.2.  Virtual Private Networks (VPNs)
3.  Determining a Domain Name
    3.1.  Domain Name Encoding
    3.2.  Access Network Domain Name DHCPv4 Option
    3.3.  Access Network Domain Name DHCPv6 Option
    3.4.  Alternative Domain Names
4.  U-NAPTR Resolution of a LIS URI
5.  Security Considerations
6.  IANA Considerations
    6.1.  Registration of DHCPv4 and DHCPv6 Option Codes
    6.2.  Registration of a Location Server Application Service Tag
    6.3.  Registration of a Location Server Application Protocol Tag for HELD
7.  Acknowledgements
8.  References
    8.1.  Normative References
    8.2.  Informative References

1.  Introduction and Overview

The location of a device is a useful and sometimes necessary part of many services. A Location Information Server (LIS) is responsible for providing that location information to devices with an access network. The LIS uses knowledge of the access network and its physical topology to generate and serve location information to devices.

Each access network requires specific knowledge about topology. Therefore, it is important to discover the LIS that has the specific knowledge necessary to locate a device. That is, the LIS that serves the current access network. Automatic discovery is important where there is any chance of movement outside a single access network. Reliance on static configuration can lead to unexpected errors if a device moves between access networks.

This document describes a process that a device can use to discover a LIS. This process uses a DHCP option and the DNS. The product of this discovery process is an http: or https: URI, which identifies a LIS.

The URI result from the discovery process is suitable for location configuration only; that is, the device MUST dereference the URI using the process described in HELD (Barnes, M., Winterbottom, J., Thomson, M., and B. Stark, “HTTP Enabled Location Delivery (HELD),” August 2009.) [I‑D.ietf‑geopriv‑http‑location‑delivery]. URIs discovered in this way are not "location URIs" (Marshall, R., “Requirements for a Location-by-Reference Mechanism,” November 2009.) [I‑D.ietf‑geopriv‑lbyr‑requirements]; dereferencing one of them provides the location of the requester only. Devices MUST NOT embed these URIs in fields in other protocols designed to carry the location of the device.

1.1.  Discovery Procedure Overview

DHCP ([RFC2131] (Droms, R., “Dynamic Host Configuration Protocol,” March 1997.), [RFC3315] (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.)) is a commonly used mechanism for providing bootstrap configuration information allowing a device to operate in a specific network environment. The bulk of DHCP information is largely static; consisting of configuration information that does not change over the period that the device is attached to the network. Physical location information might change over this time, however the address of the LIS does not. Thus, DHCP is suitable for configuring a device with the address of a LIS.

This document defines a DHCP option that produces a domain name that identifies the local access network in Section 3 (Determining a Domain Name).

Section 4 (U-NAPTR Resolution of a LIS URI) describes a method that uses URI-enabled NAPTR (U-NAPTR) (Daigle, L., “Domain-Based Application Service Location Using URIs and the Dynamic Delegation Discovery Service (DDDS),” April 2007.) [RFC4848], a Dynamic Delegation Discovery Service (DDDS) profile that produces a URI for the LIS. The input to this process is provided by the DHCP option.

For the LIS discovery DDDS application, an Application Service tag LIS and an Application Protocol tag HELD are created and registered with the IANA. Based on the domain name, this U-NAPTR application uses the two tags to determine a URI for a LIS that supports the HELD protocol.

1.2.  Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

This document also uses the term "device" to refer to an end host, or client consistent with its use in HELD. In HELD and RFC3693 (Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and J. Polk, “Geopriv Requirements,” February 2004.) [RFC3693] parlance, the Device is also the Target.

The terms "access network" refers to the network that a device connects to for Internet access. The "access network provider" is the entity that operates the access network. This is consistent with the definition in [I‑D.ietf‑geopriv‑l7‑lcp‑ps] (Tschofenig, H. and H. Schulzrinne, “GEOPRIV Layer 7 Location Configuration Protocol; Problem Statement and Requirements,” July 2009.) which combines the Internet Access Provider (IAP) and Internet Service Provider (ISP). The access network provider is responsible for allocating the device a public IP address and for directly or indirectly providing a LIS service.

2.  LIS Discovery Procedure

A device that has multiple network interfaces could potentially be served by a different access network on each interface, each with a different LIS. The device SHOULD attempt to discover the LIS applicable to each network interface, stopping when a LIS is successfully discovered on any interface.

The LIS discovery procedure follows this process:

1. Acquire the access network domain name (Determining a Domain Name).
   
   This process might be repeated for each of the network interfaces on the device. Domain names acquired from other sources might also be added.
2. Apply U-NAPTR resolution (U-NAPTR Resolution of a LIS URI) to discover a LIS URI.
   
   The U-NAPTR process is applied using each of the domain names as input.
3. Verify that the LIS is able to provide location information.
   
   The first URI that results in a successful response from the LIS is used.

A device MUST support discovery using the access network domain name DHCP option (Determining a Domain Name) as input to U-NAPTR resolution (U-NAPTR Resolution of a LIS URI). If this option is not available, DHCPv4 option 15 (Alexander, S. and R. Droms, “DHCP Options and BOOTP Vendor Extensions,” March 1997.) [RFC2132] is used. Other domain names MAY be used, as described in Section 3.4 (Alternative Domain Names).

A device that discovers a LIS URI MUST attempt to verify that the LIS is able to provide location information. For the HELD protocol, the device verifies the URI by making a location request to the LIS. If - at any time - the LIS responds to a request with the notLocatable error code (see Section 4.3.2 of [I‑D.ietf‑geopriv‑http‑location‑delivery] (Barnes, M., Winterbottom, J., Thomson, M., and B. Stark, “HTTP Enabled Location Delivery (HELD),” August 2009.)), the device MUST continue or restart the discovery process. A device SHOULD NOT make further requests to a LIS that provides a notLocatable error until its network attachment changes, or it discovers the LIS on an alternative network interface.

Static configuration of a domain name or a LIS URI MAY be used. Note that if a device has moved from its customary location, static configuration might indicate a LIS that is unable to provide accurate location information.

The product of the LIS discovery process for HELD is an https: or http: URI. Nothing distinguishes this URI from other URIs with the same scheme, aside from the fact that it is the product of this process. Only URIs produced by the discovery process can be used for location configuration using HELD.

The overall discovery process is summarized in Figure 1 (LIS Discovery Flowchart).

    -----------
   (   Start   )
    -----+-----
         |<--------------------------------------+
         |                                       |
         V                                       |
   ------^-------            ------^------       |
  /              \          /      1.     \      |
 < Next interface >-------><  Get domain   >-----+
  \              / Y  ^     \             /  N
   ------v-------     |      ------v------
         | N          |            | Y
         |            |            V
         |            |      ------^------
         |            |     /      2.     \
         |            +----<    Get URI    >
         |            |  N  \             /
         |            |      ------v------
         |            |            | Y
         |            |            V
         |            |      ------^------
         |            |     /      3.     \
         |            +----<   Check URI   >
         |               N  \             /
         |                   ------v------
         |                         | Y
         V                         V
    -----------               -----------
   (  Failure  )             (  Success  )
    -----------               -----------

2.1.  Residential Gateways

The process described in this document is likely to fail in many residential network scenarios. A fixed wireline scenario is described in more detail in Section 3.1 of [I‑D.ietf‑geopriv‑l7‑lcp‑ps] (Tschofenig, H. and H. Schulzrinne, “GEOPRIV Layer 7 Location Configuration Protocol; Problem Statement and Requirements,” July 2009.). In this fixed wireline environment an intervening residential gateway exists between the device and the access network. If the residential gateway does not provide this option to the devices it serves, those devices are unable to discover a LIS.

Support of this specification by residential gateways ensures that the devices they serve are able to acquire location information. In many cases the residential gateway configures the devices it serves using DHCP. A residential gateway is able to use DHCP to assist devices in gaining access to their location information. This can be accomplished by providing an access network domain name DHCP option suitable for LIS discovery, or by acting as a LIS directly. To actively assist devices, a residential gateway can either:

• acquire an access network domain name from the access network provider (possibly using DHCP) and pass the resulting value to devices; or
• discover a LIS on its external interface, then provide devices with the domain name that was used to successfully discover the LIS; or
• explicitly include configuration that refers to a particular LIS; or
• act as a LIS and directly provide location information to the devices it serves, including providing a means to discover this service.

As with devices, configuration of a specific domain name or location information is only accurate as long as the residential gateway does not move. If a residential gateway that relies on configuration rather than automatic discovery is moved, the devices it serves could be provided with inaccurate information. Devices could be led to discover a LIS that is unable to provide accurate location information, or - if location is configured on the residential gateway - the residential gateway could provide incorrect location information.

[I‑D.ietf‑dhc‑container‑opt] (Droms, R., “Container Option for Server Configuration,” March 2009.) might be used by an access network provider to convey configuration information to a residential gateway for use by the devices it serves. Support and use of this option is RECOMMENDED for both residential gateways and devices. Option values found within the container MUST be used after values that are directly in the DHCP response.

2.2.  Virtual Private Networks (VPNs)

A Device MUST avoid performing LIS discovery over a VPN network interface unless discovery on other interfaces is unsuccessful. A LIS discovered in this way is unlikely to have the information necessary to determine an accurate location.

Not all interfaces connected to a VPN can be detected by devices or the software running on them. In these cases, it might be that a LIS on the remote side of a VPN is inadvertently discovered. A LIS MUST NOT provide location information in response to requests that it can identify as originating from a device on the remote end of a VPN tunnel, unless it is able to accurately determine location. The notLocatable HELD error code can be used to indicate to a device that discovery has revealed an unsuitable LIS. This ensures that even if a device discovers a LIS over the VPN, it does not rely on a LIS that is unable to provide accurate location information.

3.  Determining a Domain Name

DHCP provides a direct means for the access network provider to configure a device. The access network domain name option identifies a domain name that is suitable for service discovery within the access network. This domain name is used as input to the U-NAPTR resolution process for LIS discovery.

The domain name provided in this option is one owned by the access network operator. This domain name is intended for use in discovering services within the access network.

This document registers a DHCP option for the access network domain name for both IPv4 and IPv6.

3.1.  Domain Name Encoding

This section describes the encoding of the domain name used in the DHCPv4 option defined in Section 3.2 (Access Network Domain Name DHCPv4 Option) and also used in the DHCPv6 option defined in Section 3.3 (Access Network Domain Name DHCPv6 Option).

The domain name is encoded according to Section 3.1 of [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.). Each label is represented as a one-octet length field followed by that number of octets. Since every domain name ends with the null label of the root, a domain name is terminated by a length byte of zero. The high-order two bits of every length octet MUST be zero, and the remaining six bits of the length field limit the label to 63 octets or less. To simplify implementations, the total length of a domain name (i.e., label octets and label length octets) is restricted to 255 octets or less.

3.2.  Access Network Domain Name DHCPv4 Option

This section defines a DHCP for IPv4 (DHCPv4) option for the domain name associated with the access network.

          Code   Len   Access Network Domain Name
         +-----+-----+-----+-----+-----+-----+-----+----
         | TBD |  n  |  s1 |  s2 |  s3 |  s4 | s5  |  ...
         +-----+-----+-----+-----+-----+-----+-----+----

The values s1, s2, s3, etc. represent the domain name labels in the domain name encoding. Note that the length field in the DHCPv4 option represents the length of the entire domain name encoding, whereas the length fields in the domain name encoding (see Section 3) is the length of a single domain name label.

Code:

OPTION_V4_ACCESS_DOMAIN (TBD). [[IANA/RFC-Editor Note: Please replace TBD with the assigned DHCPv4 option code, both here and in Figure 2 (Access Network Domain Name DHCPv4 Option).]]

Length:

The length of the entire access network domain name option in octets.

Access Network Domain Name:

The domain name associated with the access network, encoded as described in Section 3.1 (Domain Name Encoding).

A DHCPv4 client MAY request a access network domain name option in a Parameter Request List option, as described in [RFC2131] (Droms, R., “Dynamic Host Configuration Protocol,” March 1997.).

This option contains a single domain name and, as such, MUST contain precisely one root label.

3.3.  Access Network Domain Name DHCPv6 Option

This section defines a DHCP for IPv6 (DHCPv6) option for the domain name associated with the access network. The DHCPv6 option for this parameter is similarly formatted to the DHCPv4 option.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    OPTION_V6_ACCESS_DOMAIN    |           Length              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.                  Access Network Domain Name                   .
.                              ...                              .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code:

OPTION_V6_ACCESS_DOMAIN (TBD). [[IANA/RFC-Editor Note: Please replace TBD with the assigned DHCPv6 option code.]]

option-length:

The length of the entire access network domain name option in octets.

option-value:

The access network domain name, encoded as described in Section 3.1 (Domain Name Encoding).

A DHCPv6 client MAY request a access network domain name option in a Options Request Option (ORO), as described in [RFC3315] (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.).

This option contains a single domain name and, as such, MUST contain precisely one root label.

3.4.  Alternative Domain Names

The U-NAPTR resolution method described requires a domain name as input. The access network domain name DHCP options (Section 3.2 (Access Network Domain Name DHCPv4 Option) and Section 3.3 (Access Network Domain Name DHCPv6 Option)) is one source of this domain name.

If a device knows one or more alternative domain names that might be used for discovery, it MAY repeat the U-NAPTR process using those domain names as input. For instance, static configuration of a device might be used to provide a device with a domain name.

DHCPv4 option 15 (Alexander, S. and R. Droms, “DHCP Options and BOOTP Vendor Extensions,” March 1997.) [RFC2132] provides an indication of the domain name that a host uses when resolving hostnames in DNS. This option is used when the DHCPv4 access domain name is not available.

Alternative domain names MUST NOT be used unless the access network domain name option is unsuccessful or where external information indicates that a particular domain name is to be used.

Other domain names might be provided by a DHCP server (for example, [RFC4702] (Stapp, M., Volz, B., and Y. Rekhter, “The Dynamic Host Configuration Protocol (DHCP) Client Fully Qualified Domain Name (FQDN) Option,” October 2006.) for DHCPv4, [RFC4704] (Volz, B., “The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN) Option,” October 2006.) for DHCPv6). However, these domain names could be provided without considering their use for LIS discovery; therefore, it is not likely that these options contain useful values.

4.  U-NAPTR Resolution of a LIS URI

U-NAPTR (Daigle, L., “Domain-Based Application Service Location Using URIs and the Dynamic Delegation Discovery Service (DDDS),” April 2007.) [RFC4848] resolution for a LIS takes a domain name as input and produces a URI that identifies the LIS. This process also requires an Application Service tag and an Application Protocol tag, which differentiate LIS-related NAPTR records from other records for that domain.

Section 6.2 (Registration of a Location Server Application Service Tag) defines an Application Service tag of LIS, which is used to identify the location service for a given domain. The Application Protocol tag HELD, defined in Section 6.3 (Registration of a Location Server Application Protocol Tag for HELD), is used to identify a LIS that understands the HELD protocol (Barnes, M., Winterbottom, J., Thomson, M., and B. Stark, “HTTP Enabled Location Delivery (HELD),” August 2009.) [I‑D.ietf‑geopriv‑http‑location‑delivery].

The NAPTR records in the following example demonstrate the use of the Application Service and Protocol tags. Iterative NAPTR resolution is used to delegate responsibility for the LIS service from zonea.example.net. and zoneb.example.net. to outsource.example.com..

   zonea.example.net.
   ;;       order pref flags
   IN NAPTR 100   10   ""  "LIS:HELD" (          ; service
       ""                                        ; regex
       outsource.example.com.                    ; replacement
       )
   zoneb.example.net.
   ;;       order pref flags
   IN NAPTR 100   10   ""  "LIS:HELD" (          ; service
       ""                                        ; regex
       outsource.example.com.                    ; replacement
       )
   outsource.example.com.
   ;;       order pref flags
   IN NAPTR 100   10   "u"  "LIS:HELD" (         ; service
       "!*.!https://lis.example.org:4802/?c=ex!" ; regex
       .                                         ; replacement
       )

Details for the LIS Application Service tag and the HELD Application Protocol tag are included in Section 6 (IANA Considerations).

An https: LIS URI that is a product of U-NAPTR MUST be authenticated using the domain name method described in Section 3.1 of RFC 2818 (Rescorla, E., “HTTP Over TLS,” May 2000.) [RFC2818]. The domain name that is used in this authentication is the one extracted from the URI, not the input to the U-NAPTR resolution process.

5.  Security Considerations

The address of a LIS is usually well-known within an access network; therefore, interception of messages does not introduce any specific concerns.

The primary attack against the methods described in this document is one that would lead to impersonation of a LIS. The LIS is responsible for providing location information and this information is critical to a number of network services; furthermore, a device does not necessarily have a prior relationship with a LIS. Several methods are described here that can limit the probablity of, or provide some protection against, such an attack. These methods MUST be applied unless similar protections are in place, or in cases - such as an emergency - where location information of dubious origin is arguably better than none at all.

An attacker could attempt to compromise LIS discovery at any of three stages:

1. providing a falsified domain name to be used as input to U-NAPTR
2. altering the DNS records used in U-NAPTR resolution
3. impersonation of the LIS

U-NAPTR is entirely dependent on its inputs. In falsifying a domain name, an attacker avoids any later protections, bypassing them entirely. To ensure that the access network domain name DHCP option can be relied upon, preventing DHCP messages from being modified or spoofed by attackers is necessary. Physical or link layer security are commonplace methods that can reduce the possibility of such an attack within an access network; alternatively, DHCP authentication (Droms, R. and W. Arbaugh, “Authentication for DHCP Messages,” June 2001.) [RFC3118] can provide a degree of protection against modification or spoofing.

The domain name that is used to authenticated the LIS is the domain name in the URI that is the result of the U-NAPTR resolution. Therefore, if an attacker were able to modify or spoof any of the DNS records used in the DDDS resolution, this URI could be replaced by an invalid URI. The application of DNS security (DNSSEC) (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.) [RFC4033] provides a means to limit attacks that rely on modification of the DNS records used in U-NAPTR resolution. Security considerations specific to U-NAPTR are described in more detail in [RFC4848] (Daigle, L., “Domain-Based Application Service Location Using URIs and the Dynamic Delegation Discovery Service (DDDS),” April 2007.).

An https: URI is authenticated using the method described in Section 3.1 of [RFC2818] (Rescorla, E., “HTTP Over TLS,” May 2000.). The domain name used for this authentication is the domain name in the URI resulting from U-NAPTR resolution, not the input domain name as in [RFC3958] (Daigle, L. and A. Newton, “Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS),” January 2005.). Using the domain name in the URI is more compatible with existing HTTP client software, which authenticate servers based on the domain name in the URI.

A LIS that is identified by an http: URI cannot be authenticated. Use of unsecured HTTP also does not meet requirements in HELD for confidentiality and integrity. If an http: URI is the product of LIS discovery, this leaves devices vulnerable to several attacks. Lower layer protections, such as layer 2 traffic separation might be used to provide some guarantees.

6.  IANA Considerations

6.1.  Registration of DHCPv4 and DHCPv6 Option Codes

The IANA has assigned an option code of (TBD) for the DHCPv4 option for an access network domain name option, as described in Section 3.2 (Access Network Domain Name DHCPv4 Option) of this document.

The IANA has assigned an option code of (TBD) for the DHCPv6 option for an access network domain name option, as described in Section 3.3 (Access Network Domain Name DHCPv6 Option) of this document.

6.2.  Registration of a Location Server Application Service Tag

This section registers a new S-NAPTR/U-NAPTR Application Service tag for a LIS, as mandated by [RFC3958] (Daigle, L. and A. Newton, “Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS),” January 2005.).

Application Service Tag:

LIS

Intended usage:

Identifies a service that provides a device with its location information.

Defining publication:

RFCXXXX

Related publications:

HELD (Barnes, M., Winterbottom, J., Thomson, M., and B. Stark, “HTTP Enabled Location Delivery (HELD),” August 2009.) [I‑D.ietf‑geopriv‑http‑location‑delivery]

Contact information:

The authors of this document

Author/Change controller:

The IESG

6.3.  Registration of a Location Server Application Protocol Tag for HELD

This section registers a new S-NAPTR/U-NAPTR Application Protocol tag for the HELD (Barnes, M., Winterbottom, J., Thomson, M., and B. Stark, “HTTP Enabled Location Delivery (HELD),” August 2009.) [I‑D.ietf‑geopriv‑http‑location‑delivery] protocol, as mandated by [RFC3958] (Daigle, L. and A. Newton, “Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS),” January 2005.).

Application Protocol Tag:

HELD

Intended Usage:

Identifies the HELD protocol.

Applicable Service Tag(s):

LIS

Terminal NAPTR Record Type(s):

U

Defining Publication:

RFCXXXX

Related Publications:

HELD (Barnes, M., Winterbottom, J., Thomson, M., and B. Stark, “HTTP Enabled Location Delivery (HELD),” August 2009.) [I‑D.ietf‑geopriv‑http‑location‑delivery]

Contact Information:

The authors of this document

Author/Change Controller:

The IESG

7.  Acknowledgements

This document uses a mechanism that is largely identical to that in [RFC5222] (Hardie, T., Newton, A., Schulzrinne, H., and H. Tschofenig, “LoST: A Location-to-Service Translation Protocol,” August 2008.) and [RFC5223] (Schulzrinne, H., Polk, J., and H. Tschofenig, “Discovering Location-to-Service Translation (LoST) Servers Using the Dynamic Host Configuration Protocol (DHCP),” August 2008.). The authors would like to thank Leslie Daigle for her work on U-NAPTR; Peter Koch for feedback on how not to use DNS for discovery; Andy Newton for constructive suggestions with regards to document direction; Richard Barnes, Joe Salowey, Barbara Stark, and Hannes Tschofenig for input and reviews; Dean Willis for constructive feedback.

8.  References

8.1. Normative References

8.2. Informative References

Authors' Addresses